When designing and supporting a WLAN, however, you must be aware of potential implications, such as security vulnerabilities, radio signal interference, multipath propagation, and other issues. This chapter from Designing and Deploying 802.11 Wireless Networks explains the impacts of these problems and introduces some ways to resolve them.

As Chapter 1, “Introduction to Wireless LANs,” describes, wireless LANs (WLANs) offer tremendous benefits. When designing and supporting a WLAN, however, you must be aware of potential implications, such as security vulnerabilities, radio signal interference, multipath propagation, and other issues. This chapter explains the impacts of these problems and introduces some ways to resolve them. Later chapters explain more details on how to combat the implications.

Security Vulnerabilities

Network security refers to the protection of information and resources from loss, corruption, and improper use. With WLANs, security vulnerabilities fall within the following areas (see Figure 4-1):

Passive monitoring

Unauthorized access

Denial-of-service attacks

The sections that follow explain these security problems in greater detail.

Passive Monitoring

Wireless LANs intentionally propagate data throughout buildings, campuses, and even cities. As a result, the radio signals often go beyond the limits of the area an organization physically controls. For instance, radio waves easily penetrate building walls and can be received from the facility’s parking lot and possibly a few blocks away, as illustrated in Figure 4-2. It is possible for an unauthorized person to passively retrieve a company’s sensitive information by using a laptop equipped with a radio card from this distance without being noticed by network security personnel. A hacker, for example, might be sitting in an automobile outside a business, capturing all 802.11 transmissions using a freely available packet sniffer, such as WireShark. After capturing the data, the hacker will be able to retrieve contents of e-mails and user passwords to company servers. Of course, the hacker can use this information to compromise the security of the company. This problem also exists with wired Ethernet networks, but to a lesser degree. Current flow through the metallic wires emits electromagnetic waves that someone could receive by using sensitive listening equipment. The person must be much closer to the cable, though, to receive the signals. Thus, in terms of passive monitoring, WLANs are not as secure as wired networks.

The method for resolving the issues of passive monitoring is to implement encryption between all client devices and the access points. Encryption alters the information bits in each frame, based on an encryption key, so that the hacker cannot make sense of the data he captures via passive monitoring. An example of an 802.11 encryption process is Wired Equivalent Privacy (WEP), which was part of the original 802.11 standard ratified in 1997. WEP is fairly easy to crack, however, so it is not recommended for encrypting sensitive information. Other encryption methods, such as Wi-Fi Protected Access (WPA), offer much stronger security.

NOTE

See Chapter 13, “Security Considerations,” for more details on WEP and WPA.

Figure 4-2 Without Effective Encryption, an Unauthorized Person Can Listen in on Wireless LAN Data Transmissions

Retail Store Loses Credit Card Numbers over Wireless LAN

A garden supplies store implemented a WLAN to support communications between portable point-of-sale (PoS) terminals and servers. This allowed the store to temporarily move the PoS terminals outdoors during seasonal periods to sell garden equipment. The store did an effective job of advertising this capability, which brought more shoppers (and sales) to the store.

After the system was operational for a few months, customers of the store began contacting the store management to say that their credit card numbers had been used fraudulently soon after shopping at the store. The store found that the affected credit cards had been processed over the wireless PoS terminals, not the terminals that connect to the servers via Ethernet only. The store immediately shut down the wireless PoS terminals. After consulting with an information security consultant, the store learned that encryption was needed to keep hackers from passively monitoring the 802.11 transmissions and stealing the credit card numbers. The store had not implemented any form of encryption.

The consultant recommended that the store implement Advanced Encryption Standard (AES) encryption, which is part of WPA2 and the 802.11i standard. This keeps hackers from understanding the data in the 802.11 transmissions. The store implemented this solution and is now back to using the wireless PoS terminals without any security problems.

Hands-on Exercise: Passively Monitor a Wireless LAN

This exercise gives you some experience seeing potentially sensitive information that a WLAN transmits through the air.

Perform the following steps:

Step 1. Obtain a wireless protocol analyzer, such as WireShark (which is freely available) or other analyzers described in Chapter 14, “Test Tools.”

Step 2. Identify wireless applications to test. Choose applications that you or your organization use from wireless client devices, such as logging in to online accounts, sending and receiving e-mail, or processing credit cards, so that you can get a good idea of what a hacker can see while passively monitoring your wireless network.

Step 3. Configure the analyzer to record 802.11 frame transmissions on only the radio frequency (RF) channel of your WLAN. This helps reduce extraneous frames that the analyzer displays by filtering out frames from other channels. With fewer frames, you will be able to more easily pinpoint the 802.11 data frames applicable to your applications.

Step 4. For initial tests, turn off encryption (such as WPA); of course, you might only want to do this on a test network, separate from the operational network. Now the WLAN will not encrypt 802.11 data frame contents, which includes the information associated with the application that you are testing. This will give you an idea of what the WLAN exposes to potential hackers if you are not implementing encryption.

Step 5. While using each application you chose to test, record a packet trace with the protocol analyzer. View the recorded packet trace and look at the frame body of the 802.11 data frames pertaining to the application you are testing. To narrow down the search, try applying a filter on the packet trace corresponding to only 802.11 data frames associated with the wireless client device you are using the application from. What sensitive information, such as the user’s username and password when logging into an online bank account, are you able to find? If you are testing an e-mail application, can you interpret the contents of e-mails being sent or received via the wireless client device?

Step 6. Turn on encryption and repeat step 5. View the recorded packet trace and note the impact of enabling encryption. This allows you to see the impact of encryption and the difficulties a hacker will have when trying to acquire sensitive information from a WLAN implementing encryption. With encryption on, what sensitive information in the packet trace pertaining to your applications can you find?

With encryption turned off, you will probably not be able to find the username and password when logging into bank accounts because the session is likely encrypted between the client device and the bank’s website via Secure Sockets Layer (SSL), assuming that the online bank account implements secure web pages (HTTPS). You will likely find that many non-financial online accounts, such as hobby sites and e-mail systems, however, do not use SSL when logging into accounts. As a result, you will probably spot the usernames and passwords for those types of accounts. This is a significant issue if users have the same username and password for all accounts (which is common). The hacker just needs to monitor the user logging into a completely non-secure account, view the username and password in the packet trace, and use that username and password to log into the user’s bank account. That’s why it is a good idea to use different usernames and passwords for different online accounts. With encryption turned off, you will probably be able to find the contents of e-mail (unless encrypted by the e-mail server). Of course by turning on encryption, the WLAN will scramble (and thus hide) application-oriented information because it encrypts the frame body of all 802.11 data frames.

Unauthorized Access

If someone can connect to a WLAN, she can potentially access anything on the network, including client devices, servers, and applications, as illustrated in Figure 4-3. Some organizations do a good job of locking down servers and applications, but others do not. A hacker who can connect to a WLAN will look for backdoors and other security glitches to compromise the security of the network. For example, a hacker connected to an access point can use a TCP port scanner to implement a scan for open (unsecured) ports on servers. If one is found, the hacker has access to the port’s utilities, which might allow her to directly access sensitive information or reconfigure the network in a manner that makes it less secure (and thus easier to access more sensitive information).

One way that a hacker can gain unauthorized access to a WLAN is to stage a man-in-the-middle attack, as illustrated in Figure 4-4. There are a variety of methods to set up a man-in-the-middle attack. One is to exploit the TCP/IP Address Resolution Protocol (ARP) functions. ARP is a crucial function that a source station (such as an 802.11 radio) uses to discover the physical address of a destination station. This physical address is the MAC address, which is embedded in the client radio by the manufacturer and unique from any other client device or network component. The MAC address is analogous to the street address of your home. Just as someone must know this address to send you a letter, a sending 802.11 radio must know the MAC address of the destination. The 802.11 radio understands and responds to only the physical MAC address.

The application software that needs to send the data will have the IP address of the destination, but the sending station must use ARP to discover the corresponding physical address. It gets the address by broadcasting an ARP request packet that announces the IP address of the destination station to all the other network devices. All stations within range hear this request, and the station that has the corresponding IP address will return an ARP response packet containing its MAC address and IP address. The sending station will then include this MAC address as the destination address in the 802.11 data frame being sent. The sending station also stores the corresponding IP address and MAC address mapping in a table for a period of time or until the station receives another ARP response from the station having that IP address. This is where ARP introduces a security risk.

A hacker can fool a station by sending (from an unauthorized laptop) a fictitious ARP response that includes the IP address of a legitimate network device, such as a wireless access point, and the MAC address of the client radio in the unauthorized laptop. This causes all legitimate stations on the network to automatically update their ARP tables with the false mapping to the unauthorized laptop. This causes these stations to send future 802.11 data frames to the rogue device rather than the legitimate access point. This is a classic man-in-the-middle attack, which enables a hacker to manipulate user sessions. As a result, the hacker can obtain passwords, capture sensitive data, and even interface with corporate servers as if she were the legitimate user.

A critical security concern of IT managers is the presence of rogue wireless access points on the corporate network. A rogue access point is one that the company does not authorize for operation. The trouble is that a rogue access point often does not conform to WLAN security policies, which enables an open, insecure interface to the corporate network from outside the physically controlled facility. Figure 4-5 illustrates a scenario where a rogue access point is providing open access to the network from outside the physically controlled area of a facility.

Figure 4-5 A Rogue Access Point Can Offer an Unsecured Opening to the Network

Employees have relatively free access to a company’s facility, which makes it possible for them to inadvertently install a rogue access point. An employee, for example, might purchase an access point at an office supply store and install it without coordinating with the IT organization to support wireless printing or access to the network from a conference room. Or developers working on wireless applications might connect an access point to the corporate network for testing purposes. In most cases, employees deploying these types of access points do not understand the security issues they’re creating. These scenarios often lead to access points not conforming to adequate security practices. As a result, the corporate network is left wide open for a casual snooper or hacker to attack.

A hacker can install a rogue access point to provide an open, non-secure interface to the corporate network. To do this, the hacker must directly connect the access point to an active network port within the facility. This requires the hacker to pass through physical security, and it is easier to do than most companies assume. Nevertheless, the hacker will need to physically traverse the facility and install the access point without being noticed. It is unlikely that someone would do this unless the company has resources that are critical enough for a hacker to go to the trouble and risk of planting the rogue.

A way to counter unauthorized access is to employ an authentication system that verifies the identity of users, client devices, and access points before allowing them to operate on the WLAN. The user provides a form of credentials, such as username and password or digital certificate, and an authentication server determines whether the person (or client device) can access the network. If not, the network does not allow the client device to connect to the access point. As a result, the access point on the WLAN acts as a security gate to the network. In addition, for added protection, a company can keep all traffic on the WLAN on a virtual LAN (VLAN) that is separate from VLANs supporting sensitive applications and servers. This way, the company can limit the implications resulting from unauthorized access to only the applications and servers supporting the wireless network. A company can even go as far as keeping all WLAN traffic outside the company firewall and requiring all wireless users to implement virtual private network (VPN) client software similar to when connecting to the corporate network from public networks.

Unauthorized Access Leads to Compromise of Financial Data

A large private company in California implemented a WLAN to support enterprise mobility. The system was seemingly working great and providing significant benefits to its users. Over a year after the system went operational, the IT department noticed, through a routine network security audit, that several of its printers in the financial department had been configured to send all printed data to a file at a suspicious IP address. Unfortunately, the IT department had not locked down the administrative access ports on these printers. Even though all the details of what happened here are not known, it is likely that a hacker gained unauthorized access to the WLAN (which did not implement any form of authentication) and ran a port scan to find the open printer administration port. With the open port’s IP address (resulting from the scan), the hacker could easily log in to the administrative port and set the printer to send all print jobs to a file located on the hacker’s laptop. The printer would then continue to print on paper and also send the print data to the hacker’s laptop. Of course this would send to the hacker everything that the printer would print, such as internal goals and objectives, company sales information, employee salaries, and so on. After discovering this issue, the company promptly implemented an authentication system to disallow all unauthorized people from accessing the WLAN.

NOTE

See Chapter 13 for more details on solutions for guarding against unauthorized access.

Denial-of-Service Attacks

A denial-of-service (DoS) attack is an assault that can cripple or disable a WLAN. Wireless networks are extremely vulnerable to DoS attacks (even when using modern security mechanisms), which can cause a WLAN to slow to crawling speeds or even quit working. This causes a company that’s dependent on a WLAN to experience delays, which can be costly for some applications, such as wireless security cameras, inventory systems, and PoS terminals.

One form of DoS attack is the “brute-force” method. This type of attack can come in one of two forms:

A huge flood of packets that uses up all the network’s resources and forces it to shut down

A very strong radio signal that totally dominates the airwaves and renders access points and radio cards useless

One of the ways a hacker can perform a packet-based brute-force DoS attack is to use other computers on the network to send large numbers of useless packets to the server. This adds significant overhead on the network and takes away usable bandwidth from legitimate users. The use of a very strong radio signal to disrupt the access points and radio cards is a rather risky attack for a hacker to attempt. Because a very powerful transmitter at a relatively close range must be used to execute this type of attack, the owners of the WLAN can find the hacker through the use of homing tools.

Another form of DoS attack fiddles with the 802.11 protocols in a way that disables the network. This can be done via specialized software running on a laptop without connecting to any of the network’s access points. For example, the software can continuously send 802.11 disassociation frames to all client radios, which causes them to disconnect from the access points with which they are associated. This cuts off the client devices from the network, which of course disables them from communicating on the network, accessing applications, and so on. This method and others are well known by network culprits and published readily on the Internet.

DoS attacks are not common, and they are generally implemented over the air, thus disturbing only a small portion of a WLAN. For example, a malicious hacker with a wireless laptop might be outside a building containing a WLAN and begin broadcasting disassociation frames, but only the client radios within range of the malicious person will receive the disassociation frames and disconnect from their respective access points. Other client radios operating farther inside the building, far enough away to not receive disassociation frames, will continue operating.

Sometimes a DoS occurrence on a wireless network might not be intentional. Because the 2.4-GHz version of 802.11n resides in such a crowded spectrum, 2.4-GHz cordless phones, microwave ovens, Bluetooth devices, and other devices that use the 2.4-GHz spectrum might cause a significant reduction in WLAN performance. As a result, a company should fully investigate the use of these devices and possibly put limits on their usage before a WLAN is deemed operational.

There is not much that you can do to entirely prevent a DoS attack. A company can minimize the possibility of DoS attacks against a WLAN by making the facility as resistive as possible to incoming radio signals. This includes using directive antennas near the periphery of the building and aiming the directive side of the antenna indoors to reduce the listening capability of the antenna to signals originating outdoors. In addition, the use of RF shielding paint and window film can add significant attenuation to the exterior walls of the buildings to nearly eliminate jamming signals from outdoors. The problem with these solutions, however, is that they can be expensive and also cut off the usage of other wireless devices, such as cell phones. Also, they are not effective if a hacker somehow gets inside the building to stage the DoS attack. Of course that’s where good physical security practices come into play.

NOTE

See Chapter 12, “Radio Frequency Considerations,” for more information about implementing methods for guarding against DoS attacks.

Because of the potential harm, you must consider potential DoS attacks before launching mission-critical applications on a WLAN. If a DoS attack is even a remote possibility, think about how you will get by if the network is not available for an indefinite period of time. The benefits of the WLAN in the long term, however, will likely outweigh the disruption of an occasional DoS attack, assuming that the organization does not depend entirely on the wireless network. Something that should be put in place for any mission-critical WLAN application is a backup plan. A company should not be so dependent on its wireless network that if it goes down, everything grinds to a halt.

As with wired networks, a company should also have a “Plan B” in case the WLAN becomes unavailable because of a DoS attack. For example, a large retail store might use a wireless network to support wireless PoS terminals. In case the wireless network becomes inoperative (possibly due to a DoS attack), the retail store should have a backup plan, such as batching sale transactions for later processing when the network becomes available or when it is possible to connect the terminal to the retail system via a cable.

Be Aware of DoS Attacks on Video Surveillance Systems

When deploying a Wi-Fi video surveillance system, you must be aware that it is possible to deny the service of a WLAN. A culprit, for example, could transmit a jamming signal that holds off the video cameras from transmitting. 802.11 makes use of carrier sense multiple access (CSMA), which Wi-Fi uses to control access to the air medium. With CSMA, client radios in user devices take turns transmitting over a common RF channel. If another client radio is transmitting or an interfering (or jamming) signal is present, then all client radios within range of the jamming signal will hold off from transmitting. This situation will disrupt the video camera signals. As a result, be very careful to not depend heavily on Wi-Fi video cameras, especially if there are serious risks involved in the cameras not working!